The present invention relates to plastic injection molding apparatus and processes in which gas is injected into the mold cavity along one side of the molten resin being molded into a part and forcing it against the other mold half.
In plastic injection molding systems, the injection molds typically include a stationary half and a moving half (i.e. the core side and cavity side). The two halves are closed and clamped together to form a mold cavity for shaping articles from molten plastic materials. Plastic materials are heated into molten condition in an injection molding machine and injected under pressure through a nozzle and into the mold cavity by means of a screw ram. The plastic is allowed to cool sufficiently to harden it before the mold is opened and the articles removed.
One typical plastic injection molding problem is the formation of surface distortion or xe2x80x9csink marksxe2x80x9d on the appearance side of the final product. Sink marks are typically caused by ribs or bosses on the opposite side (or xe2x80x9cbacksidexe2x80x9d) of the part resulting from contraction or shrinkage of the plastic material during cooling. Further, warpage or part distortion can result from high injection pressures that are commonly used to fill the mold cavity, the pack out pressures, or from uneven pressure gradients resulting from the injection pressure at the gate being higher than the pressures at the outer ends of the molding. High injection pressures can also cause strain marks or warpage in the final product. Further, when ribs are formed in the molding, shrinkage differentials can cause the ribs to buckle or bend the molded part. For large molded articles where the plastic cannot flow from a single gate to the end of the molding, hot runner systems are used together with high clamping forces to hold the mold halves together. These molds are expensive and the runners may add weld lines to the products. Also, injection molding machines of a size sufficient to provide the requisite high clamping forces are expensive to acquire and costly to operate.
In gas-assisted injection molding processes an inert gas is injected directly into the melted plastic material in the mold creating hollow sections or cavities in the part. The gas, which typically is an inert gas such as nitrogen, is injected either through the plastic injection nozzle, or through one or more separate injection pin devices positioned in one or both of the mold sections. Gas-assisted injection molding processes can minimize sink marks and warpage to a certain extent since the gas can be directed through the bosses or ribs on the backside of the molded part. Such ribs or bosses, however, must be designed to be relatively thick in order to direct the gas through a channel. This is the opposite of normal design practice with plastic injection molding processes where ribs are preferably made as thin as possible in order to minimize or eliminate warpage and shrinkage, and also to decrease product cycle times.
There are some plastic injection molding processes and systems known today which inject gas on one side of the hot plastic material in the mold in order to force or compress the plastic material against the opposite side of the mold in an attempt to create a solid injection molded part with an improved surface finish. Such systems are shown, for example, in U.S. Pat. Nos. 5,439,365 and 5,273,707. These methods and systems have been known to produce injection molded parts without internal voids and with sink-free surfaces. There still is a need, however, to provide plastic injection molded processes and systems which utilize gas pressure and produce solid injection molded parts in an improved manner and with improved sink-free surface finishes.
Thus, it is an object of the present invention to provide an improved process for manufacturing solid plastic injection molded parts with sink-free surfaces and Class A finishes. It is another object of the present invention to provide injection molded parts without any internal voids, witness marks or weld lines, and does not require venting of the fluid pressure within the molded part.
It is also an object of the present invention to provide an injection molded, gas compressed, dimensionally stable, plastic part having reduced wall thicknesses, without the need for thick reinforcing ribs or internal gas cavities. It is a still further object of the present invention to provide an injection molding process that it is efficient, requires less pressure to form a part, reduces the clamping forces needed to retain the mold halves together, obviates venting, and uses at least part of the forming pressure to assist in the ejection of the finished part upon opening of the mold.
It is still another object of the present invention to provide a self-sealing arrangement during molding and curing to prevent the forming gas from either migrating around the plastic which would otherwise force the molded plastic away from the mold cavity surface used to form the finished surface, or from escaping across the part line of mold sections and outwardly from the mold cavity.
These and other objects, purposes, and advantages of the present invention will become apparent from a review of the following summary and description of the invention, when viewed in accordance with the attached drawings and appended claims.
A method and apparatus for fluid compression of injection molded plastic materials are provided to form strain-free parts without external voids and with superior finishes and sink-free surfaces. Stationary and movable mold portions define a mold cavity of the shape of the desired part. At least one plastic injection valve or sprue member is positioned in the mold for injecting melted plastic into the mold cavity. At least one gas injection device for introducing pressurized gas into the cavity is also positioned in one of the mold halves. Pressurized inert gas, such as nitrogen, is introduced into the gas injection devices and into the mold cavity following the injection of plastic. The gas operates to uniformly force the molten plastic away from one of the mold surfaces against another mold surface to form a finished outer surface on the part. It is also possible in accordance with the present invention to provide the pressurized gas only at preselected portions or areas of the backside of the molded part.
To prevent gas from escaping from the mold cavity and from migrating around the inner surface of the injected plastic to the finished outer surface of the part, seals are formed around the ejector pins and along the outer edges or ends of the part. Continuous recesses are formed in the mold surfaces encircling the ejector pins. The recesses receive thermoplastic forced into them during the injection step and the pressurized gas acts to continuously force the thermoplastic against walls of the recesses during cooling and shrinkage of the plastic material, thereby forming a seal ring that extends along the inner surface of the molding and prevents gas from escaping from the cavity and/or migrating to the other side of the part. If a moveable slide core member is utilized in the mold, for example, to create an undercut portion or area in the molded part, a small sealing groove is provided around the perimeter of the slide core member in order to prevent gas from leaking.
The thickness of the bosses or ribs formed on the inner side of the molded part have the same thickness as, or are slightly thicker than, the thickness of the outer wall of the molded part. The preferred ratio of these thicknesses is 1.4:1 to 1.0:1. In addition, the points of intersection between the bosses or ribs on the one hand and the wall of the molded part are formed without radiuses or fillets. With these features, the centers of mass of the plastic material at the intersections of the walls and bosses or ribs is positioned closer to the outer surface of the wall thereby eliminating shrinkage and sink marks.
In addition, the recesses around the gas injector devices which provide the gas seal must be sufficiently large to allow filling quickly with the plastic material, but sufficiently small in order to cool and harden quickly such that the gas pressure will provide a satisfactory seal.
In a further embodiment of the invention, the present invention can also be used in combination with internal gas assist injection molding. The gas pins can be positioned sufficiently close to a wall or fillet, or a groove can be formed in the mold, such that gas injected through a gas pin against the backside of the part also is directed or migrates into the wall or fillet in order to form an internal gas channel in the wall or fillet.